Surface coating for investment casting precision shell manufacturing

Surface coating for investment casting precision shell manufacturing

Firstly, before understanding the surface coating, let's explain the significance of each part of the shell, so that everyone can have a comprehensive understanding of the shell. We use a multi-layer shell for investment casting in precision casting. It is made by mixing binder and refractory powder into a coating, followed by immersion coating, sanding, drying, dewaxing and other processes, and then high-temperature roasting before metal casting can be carried out. Today, precision casting manufacturer Tianyao Metal will discuss in detail the surface coating for shell making.

Coatings are divided into surface layer, transition layer, and reinforcement layer, and the corresponding coatings formed are called surface layer, transition layer, and reinforcement layer. The surface layer is mainly responsible for the surface quality of the casting, while the back layer is mainly responsible for the dimensional tolerance of the casting. In the shell making operation, the purpose of sanding is to make the sand particles become the skeleton of the shell and reinforce the shell. The sand particles can also adsorb paint and solvents, and have the function of fixing the paint to stop flowing; Sanding can also disperse the shrinkage stress of silicon gel during drying to prevent large cracks. Sanding can create a rough back, which is beneficial for interlayer bonding and also improves the breathability of the mold shell. When we analyzed the detachment of the surface layer yesterday, one thing was that the surface layer sand cannot be too fine, which means it cannot form a rough back surface; There is also a back layer responsible for the dimensional tolerance of the casting. If the strength of the back layer of the mold shell is poor and the casting experiences bulging, the dimensional tolerance of the casting cannot be guaranteed. So, the reasons for these defects are actually in the middle of these simple theories mentioned above.

The function of the surface coating is to accurately replicate the surface shape and roughness of the wax mold, and finally obtain a smooth and sharp casting with direct contact with the molten metal. The surface coating is composed of binders, refractory powders, surfactants, defoamers, and some functional additives. The performance of the shell surface layer is closely related to the refractory powder in the surface coating. So, these refractory powders must have corresponding resistance to high temperature metals and good thermal shock stability; High temperature chemical stability; High temperature thermal expansion and contraction are relatively small; Should contain some coarse powder to prevent drying at room temperature and cracking of the mold shell; It should have a certain degree of breathability to avoid dewaxing and cracking, etc.

1、 Composition of surface coating. The surface layer definitely uses silica sol coating, but we usually use silica sol with small particle size (6-10X10- ³ nm), which is used separately from the silica sol on the back layer. Refractory powders generally include zircon powder, fused corundum and fused silica powder. The surface layer sand powder generally adopts a particle size of 300-325 mesh. The additives mainly include surface wetting agents and defoamers to improve the wettability of surface coatings and wax molds, and sometimes suspension agents and drying display agents are also added. The viscosity of surface coatings is generally related to the powder to liquid ratio. When using the same raw materials, viscosity increases with the increase of powder to liquid ratio. However, for products made of different materials, the powder to liquid ratio or viscosity of the surface layer may vary.

2、 The thickness of the surface coating, as shown in the figure below, is a comparison between the thickness of the shell after blowing treatment and normal production. It can be seen that the thickness after normal shell production is generally about 0.25mm. Of course, this is related to the viscosity of the surface coating. If the powder to liquid ratio of the surface layer is large, the thickness may be slightly thicker, but the difference should not be significant. The reason why thickness is mentioned is that Mr. Yamaguya Yangshu mentioned the issue of using refractory materials for the surface layer because the wall thickness of steel castings determines it. Standard grade zircon powder is used as refractory powder for surface coating of precision castings (steel parts) with a wall thickness of less than 20mm; Large components with a wall thickness exceeding 20mm must use ceramic grade zircon powder. The thicker the wall thickness of the casting, the more high-grade zircon powder should be used, otherwise the surface of the casting will be sticky and rough. Due to the fact that few domestic raw materials provide material reports for re inspection, when your product experiences sand sticking or rough surface, the first thing you need to suspect is that the surface refractory powder has insufficient fire resistance or purity.

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